In recent years, attention has been paid to a display device such as an active matrix display that uses an organic electroluminescent element (hereinafter referred to as “organic EL element”) that is self-emissive and has a wide viewing angle and high-speed responsiveness.
The display device includes: a display panel in which an organic EL element is disposed; and a drive circuit that drives the organic EL element. The display panel is configured by disposing organic EL elements in a matrix on a substrate made of glass or the like, each organic EL element including a first electrode made of Al or the like, a second electrode that is disposed oppositely to the first electrode, the second electrode being made of ITO (Indium Tin Oxide) and a metal thin film made of magnesium, aluminum, silver or the like, and a light emitting layer provided between the first electrode and the second electrode. The drive circuit is provided between the substrate and the first electrode, and is formed by using a thin film transistor (TFT) that drives an organic EL element, or the like.
Also, study has been conducted on a bottom emission type display device that provides light emitted by the organic EL elements to the outside via the substrate and a top emission type display device that provides light emitted by the organic EL elements from the second electrode side that is disposed oppositely to the substrate. However, in the bottom emission type active matrix display device, the thin film transistor of the drive circuit is formed on the substrate, and it is therefore difficult to ensure a sufficient aperture ratio.
On the other hand, in the top emission type display device, the aperture ratio is not limited by the thin film transistor or the like, and thus the efficiency of utilization of emitted light can be enhanced as compared to the bottom emission type display device. In this case, the top emission type display device provides the light to the outside via the second electrode formed on top of the light emitting layer, and thus the second electrode is required to have a high level of conductivity and a high level of light transmittance. However, in general, a metal oxide such as ITO or a thin film made of a metal such as magnesium, aluminum or silver is used as the transparent conductive material of the second electrode. Metal oxides and thin film metals have a higher resistivity than a metal layer used as a wire or the like. For this reason, a difference occurs in the wire length of the second electrode between light emitting pixels as the display panel is configured to the larger in area, which causes a large voltage drop between the end of the power supply unit and the center of the panel and creating a difference in brightness level accordingly, and as a result, the center appears dark. That is, there is a problem in that the brightness level varies depending on the position of the organic EL element on the display panel surface, which causes a deterioration in the quality of display.
In order to avoid this, it is effective to provide, for each pixel, a structure that feeds power from the low resistance wire provided at bottom to the transparent electrode provided on top.
In a display device disclosed in Patent Literature (PTL) 1, a first electrode made of a conductive material having a low resistivity and an auxiliary wire are provided separately on a substrate, a light modulation layer serving as a light emitting layer is provided on the first electrode, and a second electrode made of a transparent conductive material is provided on the light modulation layer. Furthermore, the auxiliary wire and the second electrode are connected via a buffer layer provided on the auxiliary wire.